The present invention relates to the drilling of offshore wells and particularly wells that are drilled from a floating drilling rig, i.e., a semisubmersible rig, tension leg platform or similar structure. A floating structure is, of course, subjected to wind and sea conditions that cause the structure to pitch, roll and yaw in response to these forces. When drilling from a floating structure, it is customary to use a marine conductor extending from the structure to the subsea well and held in tension. The drill string extends through the marine conductor and drilling fluid is circulated down the drill string and returns through the annulus between the drill string and conductor. The movement of the floating structure creates several problems since the marine conductor must be maintained both under tension and centered in the drilling rig. The problem of maintaining the tension on the marine conductor is solved by using the various tensioning devices. The marine conductor has been maintained center under the drilling rig by the use of hydraulic cylinders having rollers at the ends. The rollers ride along the outer surface of the marine conductor and maintain the conductor centered under the drill rig in response to movement of the floating vessel.
U.S. Pat. No. 3,528,497 discloses a system for maintaining a marine conductor centered under a drill rig through the use of hydraulic cylinders and rollers which travel along the outer surface of the conductor. The pressure in the hydraulic rams is maintained sufficient to provide the required biasing force to maintain the marine conductor centered as the floating structure moves.
U.S. Pat. No. 3,503,460 shows a similar apparatus in which opposing rams are hydraulically actuated to maintain the conductor centered under the drill rig. The patent discloses the use of pads that are moved toward and away from the conductor to maintain it centered beneath the drill rig in place of the rollers shown in the above '497 patent.
U.S. Pat. No. 3,142,343 discloses a system for centering a conductor in a drill rig in which a ring-shaped member surrounds the conductor. A plurality of springs placed circumferentially around the ring are used for biasing the ring so that it remains centered under the drill rig as the floating drilling vessel moves.